With reference to FIG. 1, in order to correctly connect synchronous machines such as electric generators 1 to an electric grid 2, a synchronizer 3 is provided that measures (via measuring transformers 4, 5) the voltage, frequency and phase of the generator 1 and grid 2, and causes a breaker 6 to connect them to each other only when these parameters are as close to each other as possible.
In other words, as shown in FIG. 4, which shows the voltage V of the generator (continuous line 10) and grid (dashed line 11) with respect to time t, the breaker is closed when the curves 10 and 11 overlap one another.
Nevertheless, during erection or maintenance of the system, there is a risk that the electric conductors 7 connecting the generator 1 to the breaker 6 or also the conductors of the measuring transformers 4, 5 are interchanged.
If this occurs, when the generator 1 and the grid 2 are connected to one another, even if the synchronizer 3 measures the same voltage, the same frequency and the same phase between the generator 1 and grid 2, the phase is actually not the same, but is switched in the worst case by 120° (three phase voltages).
In other words, as shown in FIG. 3, which shows the voltage of the generator 1 (continuous line 10) and grid 2 (dashed line 11) with respect to time, in case the electric conductors 7 or the conductors of the measuring transformers 4, 5 are interchanged, when the breaker 6 is closed, the voltage and frequency are the same, but the phase is shifted by 120° (“faulty synchronizing”).
This difference in phase causes the shaft of the generator 1 to undergo a large torque impact when the breaker 6 is closed, which tends to bring the generator shaft in phase with the grid.
Therefore, the shafts of synchronous machines such as generators are designed to withstand such a large torque impact.